The present invention relates to an electrophotographic apparatus which forms a toner image on a sheet of printing paper using an electrophotographic method or an electrostatic printing method; and, more particularly, the invention relates to an electrophotographic apparatus which develops a latent image formed on a photosensitive body using a developing agent having a magnetic carrier and a toner as main components and a first developing roller and a second developing roller rotating in directions opposite to each other and arranged along a moving direction of the photosensitive body.
A conventional electrophotographic method or electrostatic recording method, in which a latent image is formed on a photosensitive body and the latent image is developed using a two-component magnetic developing agent having a magnetic carrier and a toner as main components and two developing rollers rotating in the directions opposite to each other, is disclosed in Japanese Patent Publication No. 54-10869, Japanese Patent announcement Laid-Open No. 1-503811, Japanese Utility Model publication No. 63-15881 or U.S. Pat. No. 4,442,790.
The conventional method can prevent occurrence of unevenness in a developing direction in a high image density zone, since the developing ability is increased, but it has a problem in that unevenness remains in a direction of the developing brush in a low image density zone, particularly in a zone having an image density D smaller than 1 (one) and in a halftone image composed of mesh, crosshatch or lines, when a high resistance developing agent is used. Further, there are problems in that the size of such an apparatus becomes large since it has two developing rollers, and in that a large diameter drum needs to be used, and the printing speed is decreased particularly when the photo-response speed of the photosensitive body is slow. Furthermore, extension of the lifetime of the photosensitive body and the cleaner has not been sufficiently considered, and, accordingly, further improvement is required in order to obtain stable images for a long time.
FIG. 8(A) to FIG. 8(C) will be referred to for explaining image faults often observed in an image obtained by a conventional electrostatic latent image developing apparatus.
FIG. 8(A) is a print image of a white hollow portion 34 having a line width of 1 to 2 mm in a solid black portion 33 of approximately 3 cm square, and both the moving direction of the photosensitive drum and the relative moving direction of the developing brush are in a direction shown by an arrow in the figure. There occur a waning of the rear end portion 35, which is a phenomenon wherein the rear end portion of the solid black portion 33 is not clearly developed, and a lack of the front end portion 36 which is a phenomenon wherein the rear end portion of the white hollow portion 34 is difficult to develope. These phenomena are caused by the relative moving direction of the developing brush and an electric line of force in the periphery of the image portion. These phenomena become conspicuous, for example, when the image density D satisfies D.ltoreq.1.2, particularly in a low image density of D.ltoreq.1. When the developing ability is increased to a high image density of D.gtoreq.1.2, the phenomena become inconspicuous. However, the image of a white hollow thin line becomes difficult to reproduce, and in addition to this, the quantity of toner being consumed becomes large.
FIG. 8(B) shows an example of a print image of nearly one hundred thin lines 37 approximately 10 mm in length and 0.1 mm in width and having a spacing of approximately 0.2 mm (the total width of the thin line stripe is approximately 30 mm), and a bold line 39 approximately 10 mm in length and 3 mm in width arranged in front of the thin line stripe. In this case, there is a little waning of the rear end portion 38 in an area behind the bold line 39. This phenomenon is caused by the relative moving direction of the developing brush, an electric line of force in the periphery of the image portion and a charge induced in the carrier of the developing agent. This phenomenon becomes conspicuous, for example, when the image density D is smaller than 1 (one). When the developing ability is increased to a high image density, the phenomenon becomes inconspicuous. However, the image of the thin line stripe becomes difficult to reproduce.
FIG. 8(C) shows an example of a defective image occurring when a solid black portion 41 in a mesh or crosshatch portion 40 having 100 to 600 dots/inch is recorded. In this case, in addition to a waning of the rear end portion as described above, there occurs a decrease of the image density in the peripheral portion of the solid black portion or a fringe image 42 blanked in white, which is conspicuous in the rear portion compared to both side portions in the lateral direction and the front portions. This phenomenon is caused by the relative moving direction of the developing brush and an electric line of force in the periphery of the image portion. This phenomenon becomes conspicuous, for example, when the image density D is smaller than 1 (one). When the developing ability is increased to a high image density, the phenomenon becomes inconspicuous. However, the image of the mesh or crosshatch is apt to be crushed.
The inventors of the present invention have studied these image faults from various aspects and found that the image fault closely relates to a circumferential speed ratio between the first developing roller and the photosensitive body and a circumferential speed ratio between the second developing roller and the photosensitive body.